Pipelines operated at elevated temperatures or internal pressures must be designed to accommodate longitudinal expansion. Above-ground pipelines typically incorporate expansion loops of sufficient number and flexibility to accommodate this longitudinal thermal expansion. Buried onshore pipelines often include concrete anchors to control movement at the pipeline ends and bends. Such loops and anchors are not typically utilized in subsea operations because of the method used to lay subsea pipelines. Subsea pipelines are typically fabricated one segment at a time aboard a pipeline-laying vessel. As each segment is added, the vessel moves forward and the pipeline follows a descending path to the seafloor. The suspended pipe span between the vessel stern and the seafloor is typically supported by a stinger attached to the vessel stern and axial tension applied to the pipe. Applying this tension to a pipeline incorporating expansion loops will exceed the elastic stress limits of a typical expansion loop. Typical expansion loops would also not pass easily through the laybarge pipe tensioning machines or stinger.
Accommodation of thermal expansion is further complicated because subsea pipelines in shallow water are typically trenched into the seafloor to provide stability and to protect from damage by fishing equipment and anchors. Installation of subsea anchors to control longitudinal expansion at the ends of the pipe is often uneconomical. Additionally, restraining expansion results in increased longitudinal compressive forces in the pipe. Similar high compressive forces develop away from the ends on long subsea pipelines due to the frictional restraint provided by the pipe resting on the sea bed. The maximum longitudinal compressive force which a trenched pipeline can sustain without buckling depends on the pipe properties, initial configuration and restraint to movement provided by the surrounding soil. Conventional subsea pipeline installation and trenching methods result in pipe elevation deviations. The pipe forms an overbend when laid over a high point on an otherwise flat trench bottom. If the weight of the pipe plus soil overburden is less than the uplift force caused by compressive pipe forces at the crest of an overbend, the pipe will lift up. This is referred to as an upheaval buckle and can be a cause of pipeline failure. Once an upheaval buckle initiates, pipe forces in from both directions, and the buckle will grow. As the buckle grows, any overburden on the pipe at the point of the buckle is also reduced, resulting in an acceleration in the growth of the buckle. When a method to accommodate thermal growth causes the pipe to maintain compressive forces, the pipe will require routine and detailed inspections to ensure that buckling has not started. Design criteria which rely on maintaining compressive forces within the pipeline are described in OTC paper No. 6335 and by Pedersen, et al. in "Upheaval Creep of Buried Heated Pipelines with Initial Imperfections," Marine Structures, Vol. 1, pp. 11-22 (1988). It is preferable to provide a method to accommodate thermal/pressure expansion in which compressive forces are controlled, rather than one which constrains movement and maintains compressive forces within the pipe.
A method which limits compressive forces due to thermal expansion is described in OTC paper No. 6334 by Craig, et al. In this method, a pipeline is laid onto the ocean floor, and then water of a temperature of about the operating temperature is passed through the pipeline. Because the pipeline is not trenched, it will snake laterally to accommodate thermal expansion. The bends which accommodate the thermal expansion are uncontrolled and are assumed to be elastic. The pipeline is then trenched into the seabed while in the expanded mode. This method relies on the friction between the pipeline and soil to maintain the pipe in the expanded configuration. This friction induces tension in the pipeline when the pipeline is cooled and reduces compressive forces when the pipeline is reheated. The pipeline will contract somewhat when cooled, and to the extent this movement is not reversible upon reheating of the pipeline, compressive loads will be placed upon the pipeline. Heating the pipeline after it is laid and while it is being trenched can also be expensive and time consuming. Further, the expansion is not accomplished in a controlled fashion, and could result in a combination of sharp bends and long straight runs. It would be preferable to provide a method to accommodate thermal expansion which does not rely on frictional forces against soil to maintain tension and which accommodates thermal expansion in a controlled manner.
OTC paper No. 6335 summarizes other methods to accommodate thermal expansion in trenched subsea pipelines. Among them are reducing wall thickness of the pipe to reduce compressive forces, backfilling the pipeline trench with rocks, and using a pipe within a casing pipe configuration. Each of these methods are relatively expensive or of questionable effectiveness.
It is therefore an object of the present invention to provide a method of accommodating thermal expansion within a heated subsea pipeline and a subsea pipeline capable of operating at elevated temperatures. It is a further object to provide such a method and pipeline wherein frictional forces against soil are not required to retain tension when the pipeline is not in a heated state. It is another object to provide such a method and pipeline wherein the expansion is accommodated at frequent and relatively uniform intervals along the length of the pipeline.